In the current model of taint, only symbols (subclasses of SymExpr) can "truly" carry taint: the whole taint thing is about mapping symbols to taint kinds. It makes sense that only rvalues can be tainted, while memregions represent lvalues (segments of memory).
A memory region is, by definition, said to be tainted iff the value of the start of the segment (the numeric value of the pointer to the start) represented by the MemRegion object is a tainted rvalue. Hence, a memory region can be said to be tainted in the following cases:
1. It is a SymbolicRegion, and its parent symbol is tainted.
2. It is an ElementRegion, and its symbolic index value is tainted.
3. It is a sub-region of another tainted region.
The helper function ProgramState::addTaint(const MemRegion *) is a simple wrapper that works only on symbolic regions directly, and adds taint to its parent symbol. It does nothing, and in fact cannot do anything sensible, if the region supplied is not of SymbolicRegion class.
I believe that the problem you are encountering is that you are trying to add a taint on a region that cannot actually be tainted (eg. VarRegion). You can see what your region is by dumping 'loc'.
Note, however, that any symbol that is based on a tainted region (eg. SymbolRegionValue of a tainted region) becomes tainted itself automatically, even if it's not explicitly included in the taint map. But it doesn't mean that the value of a tainted region is necessarily tainted; the value of a tainted region can be easily overwritten with a trusted value.
So if your checker needs to add a taint to a MemRegion that does not fall into one of the three categories mentioned above, then there must be something wrong with the approach. Say, VarRegion itself cannot be tainted; there's nothing wrong with a pointer to a well-defined variable inside the program, this pointer value is not something that the attacker can alter to produce unwanted results. It's a concrete region, we know everything about it, while taint is all about values obtained from external, untrusted sources.
If, on the other hand, your MemRegion falls into category 2 or 3, and tainting the region (or, in other words, the pointer to that region) is what you truly want, then you'd need to manually unpack the region and taint the particular symbols inside it, either the index symbol, or the symbolic base (if your region is a sub-region of a symbolic region, see MemRegion::getSymbolicBase()).